Because the water/cement ratio is a prime determinant of slurry properties, one objective of any slurry design is to keep the annular column of suspension homogenous until the cement has set. Microscopically, there are two extreme situations possible for a cement slurry; either it is dispersed, i.e., the particles are separated by repulsive forces between negatively charged surfaces, or it is flocculated, i.e., the particles form an interconnected structure held by electrostatic bonds between unlike charges. When the slurry is fully dispersed, it becomes unstable and shows sedimentation, resulting in large vertical density gradients while no supernatant water may be present. Conversely, when it is flocculated, free water may form with the suspension below remaining homogenous. Practically, intermediate situations where only a fraction of the particles are flocculated are sought, leaving the slurry stable but not too viscous.

In this paper, the first part describes and provides mathematical models for the two basic phenomena which may create a density gradient in a cement slurry: (1) the subsidence of a network of flocculated particles; (2) the sedimentation of individually dispersed particles. The second part presents slurries with various physical properties which display actual density gradients, and correlates these gradients with the models. The final part describes a newly developed anti-settling additive, which facilitates the design of stable cement slurries, including field case histories.

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